Hyperloop Physics 101 With Elon Musk

An artist's impression of the Hyperloop pod Courtesy Elon Musk

On Monday afternoon, Elon Musk, chief executive officer of SpaceX and Tesla Motors, unveiled his much anticipated Hyperloop. We’ve published all the details on this super-fast new mode of transportation, along with an interview with Musk. Still, this event offers an opportunity for a little seminar with Elon Musk, physicist. For those of you ready to geek out and then some, here’s Musk—University of Pennsylvania physics undergrad, Stanford applied physics materials science PhD-dropout, and SpaceX rocket designer explaining some of the fundamental properties of the Hyperloop.

Tell us about the basics of the design. You have pods, with skis on the bottom, zipping through tunnels put under low pressure. Why did you pick this design?
The pods will ride on air bearings. The pod produces air, and it’s pumped out of little holes on these skis. This is something that is used quite a bit in industry. You can move huge, heavy objects with very low friction, using air bearings. In the consumer sense, people would be familiar with air hockey tables, except in this case the air bearings are being generated by the pod itself, as opposed to the tube.

You don’t want the tube to be expensive. Because the tube is so long, you want the expensive stuff to be in the pod.

Ahead of the release of the details of the Hyperloop, some people said what you were proposing sounded impossible because it would require too much energy to get something through a tube at such high speeds and long distances.
There were guys questioning the energy that would be required to move the air and the pod. They didn’t quite appreciate that it’s not the air that is moving the pod. The pod is accelerated to velocity by a linear accelerator, which is basically a rolled-out electric motor. The air in the pod is going maybe 200 to 300 miles an hour, and it is low-density. So some of these guys were thinking: ‘Oh, the air is sea-level density, and the air itself will be the thing that pushes the pod.’ But that is not the case.

You do want to have a continuously circulating loop of air so that you are not losing energy by letting the air slow down. But it is more efficient to have the pod go faster than the air. If you just try to pump air—particularly at sea-level pressure—through what is effectively a 700 mile loop, the energy required would be extremely high if you wanted that air to go fast because of friction against the side walls of the tube.

How would the linear accelerator work that gets the pods going?
It’s actually a linear electric motor. It’s a very basic thing. They have been around for a very long time. The air skis in the pod would have a thin row of magnets—you don’t need much. The linear motor would electromagnetically accelerate the pod. It would be just below where the skis are. It just creates an electromagnetic pulse that travels along the tube and pushes the pod to that initial velocity of 800 miles per hour.

About half a dozen re-boosts would be needed between San Francisco and L.A., but the linear induction motor size needed for re-boosts is much smaller than the initial one.

How would you slow down?
When you arrive at the destination, there would be another linear electric motor that absorbs your kinetic energy. As it slows you down, you put that energy back into a battery pack, which then provides the source energy for accelerating the next pod and for storing energy for overnight transport.

The solar panels would be laid on top of the tubes. You would store excess energy in battery packs at each station, so you could run 24-7.

Have you looked at some of the drawings that Rand came up with or the ET3 concept?
I wasn’t familiar with either of those before, but I have learned about them now. The Rand approach was not practical because it relies on a pure vacuum and it’s a tunneling approach. You have to tunnel some phenomenal amount of earth. It would be really expensive. You would have to make tunnels 100 times cheaper than they currently are.

The ET3 approach uses essentially a vacuum tube above ground. It is not exactly a vacuum tube because they never technically get to a vacuum. But the level they are talking about is not practical. It is worth noting that they have been promoting their thing for 20 years and still don’t have even a little demonstration prototype. They are also relying on superconductors, and you would have to carry liquid nitrogen on the pods. These things are not without their challenges. Let me put it that way.

How many people helped you with the Hyperloop design?
There are probably about a dozen people, between SpaceX and Tesla, who I have asked to vet some of the ideas and make sure that my calculations are right and contribute any ideas that they can to the mix. One of the things they very quickly pointed out was that we don’t want to have a wind speed in the tube that is too fast. The energy taken up by the friction of the air column against the tube will be prohibitive, which is exactly what other people have pointed out. If the column of air is too dense or moving too fast, you just generate too much friction.

There are some ideas floating around about using linear acceleration to help put rockets into space. Could the Hyperloop technology be applied to SpaceX?
No. People who say this have no idea what they are talking about. Even if you were hell-bent on doing ground-based rocket booster acceleration, you certainly wouldn’t do it in a bloody tube! The best way to do that would be to mount a humongous linear induction motor to the side of a steep solid rock cliff on a tall mountain that is near the equator. That would be extremely complicated and expensive and you would always have horrible logistics issues. Or you could just increase the size of the booster by 10 percent to 20 percent and not worry about all that.

This is one of my pet peeves—along with space solar power, which is such an obviously dumb idea that it blows my mind that some smart people would suggest it. If anyone has a vested interest in space solar power, it would have to be me.

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